Retroactive valve apparatus



Feb. 13, 1940. .1 RoBlNsON RETROACTIVE VALVE APPARATUS 2 sheets-sheet 1 Filed Aug. 8, 1939 Feb. 13, 1940. J. RoBlNsoN `2,1903-,243

, RETROACTIVE VALVE APPARATUS File@ Aug. 8,'1939 2 sheets-sheet 2.

30 'k .if .a5 .y l l UFL/Pff l '1f f ,if il j l B2i A ze Patented Feb. `13, 1940y UNITED STES-l aimais -axiraoac'rivn VALVE ArPArcA'rs James Robinson, London, England Applicaties August 8,1939', serirNo. "289,059

AIn, Great Britain December 14, 1937 i'. 6 illaims.

, This invention relates to retroactive valve apparatus. In `such apparatus' it has been `proposed to apply` retroactionl'thrcughfrequency selective means. such that negative retroaction4 is obtained for some frequencies of an applied` band so as toattenuatethe signals of these frequencies and to produce no substantial eect or to produce positive retroaction for applied signals of other frequencies within `the band.V

The present invention is concerned with a retroactive valve apparatus for an applied band ofv frequencies, for examplaras described in my British Specification No. 466,415, in which there is provided a frequency changer comprising ,a valve oscillator and mixing device, to which mix' ing device wave signals vandvlo'cal oscillations l are applied, and retroactive means for applying a portion of the beat frequency sigr'lal output ,derived from the, frequency changer to the os,` "cillator tofchange the signal `amplitude `as appliedto the `mixing device. In this manner there is obtained an effectiveincrease in selectivity which may be further increased by employing a highly selective device/such as a piezo- .l electric crystal circuit iin the` said. retroactive` means. `According to the present` inventionthe retroactive means aforesaid is suchas topro-,z vide,` substantial negative retroaction` for a major part ofthe band of frequencies and a small4 negative, zero or smallpos'itiveretroaction (referred to collectively hereinl as a` lessnegative retroaction) for a narrow intermediate part` of said'band of frequencies at the linputv of the frequency changer. i

Itwill be understood Vthat by negative retro-W action is meant retroaction in` which the phase difference between the applied signals and the signals fed back is more than 90 and by posiu tive retroaction ismeant retroaction in vvhichy-V necessary conversion of the frequency back to the original frequency of the applied signals bef l ing obtained by means of-the frequency-changer` the. phase difference is less than90. Y l

In the preferred form of the invention the retroactive means aforesaid `comprises a bridge circuit having a' selective circuit or device in one arm and a resistance in the opposite arm. Thus, signals passing the resistance arm of the bridge are opposed to'signals passing the resonant arm, sothat the resultant response ofthe bridge circuit is small atl or near the resonant l frequency and is large for higher `and lower frequencies. The Hfrequency selective device of the bridgecircuit may consist of a piezo-electric device and, in this case, a condenser is provided in the opposite arm in order kto balance,` the 'capacity of the piezo-electric device either coml-` pletely or to a greater orlesser extent. u l

vtive selectivity apart from that due to anyin-` crease in the amplitudeof thecarrier-frequency vceiver shown in Figure. 1,

One application of the invention is to receivers for modulated carrier wave signals.` In thisca'se frequency by, say, to 5,000 cycles and, in consequence, there is provided an'increase in effecwhich may occur due to the. retroaction. Thus,

a greater selectivity with stability can` be obtained. Y Specific circuit arrangements of receivers embodying the invention are shown by Way of exk amples in the accompanying drawings-in'which:

` Figure 1 shows a circuit arrangement of` a superheterodyne receiver, for modulated carrier-'Wave signals,

Figures 2 and '3 show curves representing'the. operation of theretroactive means of the re- I Figures 4 and 5 show modifications rangement shown in Figure 1, While' Figure 6 shows a lcurverepre'sentingthe op- `ofV the. ar-

`erationof the retroactive means in Figure 5`.

or by an additional frequency-changer, for in-` stance as described 4in my British specification l 45 Referringto Figure 1, the receiver comprises kapre-selector'circuit or circuits, indicated at 30, a local oscillator 3l associated with a detector or mixer 32,any intermediate frequencyamplierk33 and a second detector 34. A portion of the intermediatefrequency signals taken, for example, from the output of the mixer .32 is applied through a feed-back circuit as hereinafter described to. another mixing device v36 and combined with a portion of the'output of 55` the local oscillator 3l to reproduce the incoming frequency signals. ,These reproduced signal vfrequencies are applied to theselector circuit 3B of the receiver with such phase `relationship as to provide negative .retroaction for at least the major part of the frequency band passed-by the circuit 30.

The feed back circuitl comprises an electrical bridge having a piezo-electric device lin one arm and a condenser 2G vand resistance 21 in parallel in the opposite arm, the remaining arms of the bridge being constituted by a tapped inductance 28 which is tuned bya condenser 29. An output from the mixer 32 is applied to the inductance 28 through the coil'35 coupledthereto and the output of the bridgetaken from thje junction of the piezo-electric device ,25 4and resistance 271 and a tapping on the'inductance 28` is applied to the mixer 36. f

The tuned circuit constituted by the inductance 28and the condenser 29 is broadly tuned vto the intermediate frequency of 'the applied modulated carrier-wave signals and the piezol electric device, which has a sharper characteristie, has the same resonant frequency. The

piezo-electric device provides an output having a sharp peaked resonant characteristic, while l the output from the resistance" is of comparavtude due tothe negative retroaction. There is 30 vtively uniform amplitude for the'applied band ofv frequencies and opposes the output from the piezo-electric device. I

YIt will be understood, therefore, that by varying the value of the resistance 2T the resultant `input to the mixer 36 mayhave different frequency-amplitude characteristics, so that there may be obtained a good control ofthe final amplitude ofthe carrier-frequency as compared with the amplitude'of associated frequencies, as aforesaid. For example, the output from the resistance, Vas represented by the line I8 in Figure 2, may belarger than the maximum response of the 'piezo-electric device of which the frequencyamplitude curve is indicated at I9. The resultant .response which isk applied to the mixer 3b is represented bythe curve 2i) which thus has a minimum response at the resonant frequency indicated at f and a substantial response for frequencies at either side of the resonant frequency.

After undergoing a frequency change at the mixer 36 this response is applied to the circuit 30and although the carrier-frequency of the signals in this circuit is not appreciably affected due to the low amplitude of the retroaction at this frequency, the frequencies at either side of the carrier-frequency are reduced as regards amplithus obtained at the circuit 30 incoming signals which" are modied by the negative retroction to produce a corresponding effective selectivity, though without amplification.

The provision for a 'slight negative retraction approximating tothe resonant frequency, thereA being minimum values at two frequencies f1 and f2 at either side .of the resonant frequency. A

larger response is further obtained for other frequencies of the band as indicated by the parts 22 of the response curve. It will be understood in this case that, by applying negative retroaction as regards the frequencies of the band included in the parts 22 of the response curve shown in Figure 3, there will be obtained` positive retroaction for frequencies at and adjacent the carrierfrequency. Thus, in this manner the carrierlfrequency may be augmented by retroaction in addition to the amplitudes at other frequencies `being reduced as hereinbefore described for obtaining selectivity.

. It will be understood, therefore, that by varying thevalue of the resistance 21 the amplitude of the carrier-frequency may remain unaltered or may be raised or lowered as required and, if

y desired, this resistance may be varied automatically under the control of the signals in order toprovide an automatic volume control for the receiver.

The condenser 26 is preferab-ly'of lsuch valuef as to balance the capacity of the piezo-electric device 25 in order to obtain a substantial symmetrical response characteristic for vthe bridge circuit. In some circumstancesit is desirable to unbalance the .bridge circuit asregards capacity or inductance so as to modify the change of phase at the carrier-frequency with Irespect to' other adjacent frequencies. This" unbalancing of the bridge circuit may be controlled automatically according to the strength of the signals in. order to provide an automatic volume control.

Furthermore, instead of employing the complete x bridge circuit in the intermediate stage of the receiver it may be partly in .the intermediate frequency stage andpartly in another stage separated therefrom by a frequency-changer.

One example of an arrangement of this nature is shown in Figure 4. The receiver comprises a pre-selector 3i?, local oscillator 3l, 'mixer 32,

vtan intermediate frequency amplier 33 and a second detector and output stage One feed `back circuit comprises inductance 35, the tuned circuit 28, 29 and the piezo-electric crystal 25 and balancing condenser 26 which are connected withr a second mixer 36 where the energy fed back is reconverted to the original signal frequency by means of the localoscillator 3l. 'I'he second feed back circuit comprises an absorber 3l coupled with the routput of the pre-selector 30 and feeding an amplierl 38 the output ofwhich is connected with the pre-selector 39. In operation-- the piezo-electric crystal is tuned to the inter-` mediate frequency ofthe receiver andthe feed back in this part of the circuit is arranged to be positive. to eliminate the wanted carrier-frequency and the feed back here is in the negative direction.Y

By adjusting the magnitude of the feed back in the two circuits effects similar to that described with reference to Figure l may be obtained.

Instead of employing a piezo-electric device as selective means, for the retroactive apparatus, other selective means can be employed such as, for example a series tuned circuit. It rwill be also vunderstood that the bridge circuit aforesaid provides a response which isjcharacteristic of an absorption circuit and, in carrying out the in-` vention, other forms of absorption circuit may be employed. For example as illustrated in Figure 5- two piezo-'electric devices 39, 40 with different resonantk frequencies may be employed,l one in each of vtwo opposite arms ofthe bridge,4

Il f1 and unbalanced by means of a condenser 4l solas The absorption circuit 31 is arrangedy passing below the resonant frequency o-f the low-f' er frequency device and above the resonant areogma to provide ,aminimumresponse atan intermediate frequency," i. v,

As described in myco-pendiirigV application for U. S.; LettersPatent Serial 1No.v 232,194, the input i toithe'twov piezo-electric devices is f applied .inn v opposed phase by means of the centre tapped in.- ductance Eiland, sincethe change in phase occurring in a signalwhenituned through the` reso-` nantl frequency of such a device'is very sudden,

, the outputs from the two devices may beregarded .l asbeing in phase for energy of frequencies be` tweeny their resonant frequencies, while energy frequency `of the higher frequencydevice` will also be in phase but ofv opposite sign. Since a piezo-electric device will have an inductive reactance for signals of a frequency above the A,resonant frequency and a capacitative reactance for signals of a frequency below the resonant frei quency, the addition of a condenser in shunt istic of the form shown in Figuredmay be ob` therewith will increase the energy transfer` below the resonantfrequency `and decrease theen'ergy transfer above Vthe resonant frequency. If,there fore, the condenser is connected in shunt with the device of lower resonant frequency, a charactertained wherein the peaks at f1 and f2, arevat the resonant frequencies of thedevices 39 and lil respectively. rAdjustrnent of the size of the condenser determines thev relative `response ofthe l s circuitto signalsof a frequency between f1 and f2r and aboveland below these frequencies, and, if

desired, `a' relatively complete suppression of energy between f1 and f2 may be obtained, and owing to the dierencein phase of energy passing the circuit betweenand outside of the peaks at f1 and fathe feed back may be arranged to be negative for all frequencies except within the range fi-fi. The feed back kwithin vthis range will be positive but by a suitable design cf4 the circuit the energy transfer in this range may be i reduced to a very` low value.`

The arrangement may thus be used to give the pre-selector circuits 38,in effect, a band pass characteristic of a very satisfactory shape irre' spectiveofthe frequency t0 which these circuits are tuned. Furthermore, the retroactive circuit may comprise one or more parallel tuned circuits, for exampleya plurality of such circuits, ar,- ranged in series, with or `without thebridge circuit hereinbefore described, s y 's It will be` appreciated ,that the invention is applicableto the selective treatment of aband of frequencies, for` instance, constituted by a carrier-wave and one or both' side-bands, fini transmitting, repeating or rreceiving apparatus.` The apparatus according to theinventionmay also be used tosuppress the carrier-frequency of a modulated carrier-wave transmission by con-' necting a piezo-electric crystal tuned tothe frequencyof the4 carrier-wave in parallel with the condenser 29 of Figures 1, 4 and 5.'v The apparatus in accordance with this invention may also be used to change the phase of the carrierfre` the beat frequency signal output from the frequency changer to the oscillator to provide at the -frequencies y `resonant frequencies of` the crystals in said abinputl l `of n .thefrequency` "changer: `a substantial negative retroaction fora major part ofthe band of frequencies andra less negative retroaction for a narrow intermediatevpart offsaidfband of frequenciesrx x i,

`2. Afretroactivevalve amplifier 1 comprising aA y frequency'lchanger.having a local` oscillator and.` mixing device to which device locaioscillations and `signal frequencies rare applied to form i a beat j frequency, retroactive means comprising a bridge.' vcircuit having ,aselective' device one arm` and a- 1 resistance in` theH opposite' arm, `meansfor` y feeding a portion of said beat frequency to said bridge circuit and means for combining the output of said bridge circuit with localoscillations andfor feeding the resultant signal frequency to saidfrequency changer to provide at the input circuit of `saidfrequency changer a substantial negative retroaction at all signal frequencies except the frequency band of the wanted signalfrequency. I s l 3. A retroactive valver amplifier comprising a frequency changer having a local oscillator and mixing device to which device local oscillationsv and-signal frequencies are applied to form a beat frequency, retroactive means comprising a bridge circuit having a piezo-electriccrystal in one arm opposite arm, means for feeding a portion of said beat frequency tothe input of said bridge circuit, a second mixing device to whichis applied the output of said bridge circuit and oscillations from said local oscillator and means for feeding the output of said second mixerto the input of said 'frequency changer toprovide a substantial nega- `tive retroaction at all signal frequencies except the frequency band of the wanted signal.` 4

y and a resistance shuntedby acondenser in the 4. A retroactive valve amplifier` comprising a frequency changer having a local oscillatorand y mixing -device to which device `local oscillations and signal frequencies are applied to form a be'at frequency, an absorption circuit comprising two {piezo-electric crystals having different resonant f frequencies arranged in opposite arms of a bridge and a condenser `connectedin parallel `with the crystal of` the higher frequency,.rneans for feed-v ing a portion of the beatfrequency to the input n of `said absorption circuit, a second mixing device l to which is applied the output of said absorption circuit and oscillationsfromsaid local oscillator,

and means forfeeding the output of said second mixer to the input of said frequency changer to,

provide a substantial negative retroaction at all except those lying between the sorpticn circuit. f

5. A retroactive valve amplifier comprisingV a frequency changer having a local oscillator and mixing device to which device local oscillations and signal frequencies are applied to form a -beat frequency, a selective circuittuned to said beatl frequency, a second mixing device to which `beat l s frequencies from said-selectivecircuit and oscilrlations from said local oscillatory are fedyand n means for feeding the'output `of said second mixer to the input of saidrfrequency changer so as to provide positiveretroaction, an absorption circuit tuned tol suppress the wanted signal frequency to which signal ,frequenciesare fed, van j amplifier the input of which is'coupled with kthe output'of said absorptioncircuit and the output of which is fed to said frequency changer to pro- Videv a substantial negative retroaction at all lfrequencies except the frequency band of the wanted signal.

' 6. In retroactive valve apparatusxf'orfan applied band of signal frequencies, the combination with a valve having input and output circuits, of

itative means in parallel with the mechanicall resonator of higher frequency, whereby the energy output of said absorption circuit at frequencies intermediate resonant frequencies of said mechanical resonators is reduced to a relatively loW value, and means for feeding the out-` put of said absorption circuit to the input circuit of said valve so as to provide substantial negative retroaction at all signal frequencies except those intermediate the resonant frequencies of said mechanical resonators. i

JAMES ROBINSON. 

